Ischemia occurs when the blood supply to the heart muscle is temporarily or permanently reduced, such as may result from the occlusion of a coronary artery. This occlusion may lead to local ischemia or infarction of the heart muscle. Ischemia may also occur over large sections of the heart muscle due to conditions such as cardiac arrest, heart failure, or a variety of arrhythmias. The ischemic event can be of the so called “silent type” described in medical literature (e.g. not manifesting itself in terms of symptoms experience by the patient or obvious external indications). The event can also be chronic with continuously evolving symptoms and severity due to underlying heart disease, or very abrupt and possibly even fatal due to infarction of large enough area of the heart to cause a large myocardial infarction.
The ischemic event often causes the performance of the heart to be impaired and consequently manifests itself through changes in the electrical (e.g. the electrocardiogram signal), functional (e.g. pressure, flow, etc.) or metabolic (e.g. blood or tissue oxygen, pH, etc.) parameters of the cardiac function.
The conventional approach to detection of MI/I is to analyze the electrocardiogram (ECG). An ischemic event results in changes in the electrophysiological properties of the heart muscle that eventually manifest themselves as changes in the ECG signal. The current state of the art is to record these ECG signals from the body surface using amplifiers and associated instrumentation. A standardized set of electrodes in an arrangement known as a 12-lead ECG has been developed. The conventional approach to the detection of ischemia and infarction relies on analysis and interpretation of characteristic features of the ECG signal such as the ST-segment, the T-wave or the Q-wave. Computer-based technology has been employed to monitor, display, and semi-automatically or automatically analyze the ischemic ECG changes described above. The present technology includes ECG machines used in doctor's office, portable ECG machines known has Holter recorders, bedside monitors with displays, and sophisticated computer-based system for automatic analysis of the ECG signals.
Technology exists for providing therapy once ischemia is detected. The most common approach involves thrombolytic therapy (by external infusion of drugs such as TPA or streptokinase) or opening of the blocked vessels using a variety of angioplasty catheter devices. In the event that ischemic condition results in malignant arrhythmia or arrest of the heart, an external defibrillator may be used to shock the heart and restore the cardiac rhythm.
Technology also exists for implanting therapeutic devices for treating electrical conduction disturbances or arrhythmias of the heart. These devices include implantable pacemakers, cardioverters and atrial and ventricular defibrillators, drug infusion pumps as well as cardiac assist devices. The implantable devices typically use intracavitary leads to sense the electrogram (EGM) and then provide electrical therapy (pacing or defibrillation) or mechanical therapy (pumping blood). These devices sense the EGM and then utilize the features, such as improper conduction (in case of a pacemaker) or a fatal rhythm (in case of a defibrillator), or simply timing (to coordinate mechanical pumping). Notably, these devices do not specialize in the task of detecting, alerting the patient or treating ischemic heart disease.
Ischemia detection and analyses are usually done manually by the expert cardiologist or by computers employing algorithms to detect ischemia-related changes in the ECG signals. The preferred features of the ischemia detecting computer algorithms are the ST-segment and the T-wave. These features show elevation, depression or inversion of these ECG signals associated with ischemia. The computer then carries out a careful measurement of the degree of elevation/depression in a specific lead. By identifying ischemia dependent changes from specific leads, the ischemic event is attributed to a specific region of the heart.
The current approach to diagnosis is that after an ischemic event is perceived by the patient, they contact medical personnel such as the “911” system or their personal physician. Within the clinical setting, the patient is often monitored using a short recording of the ECG signal which may be interpreted by a physician. Alternately, the high risk patient may be continuously monitored at the bedside in a cardiac intensive care unit. Therapy may include using drugs such as TPA, use of catheters for angioplasty (opening the blocked coronary vessel using a balloon or laser), or providing life support back up such as defibrillation.
The aforementioned cardiovascular medical monitoring technology and medical practice have several significant drawbacks in regard to the detection and treatment of coronary ischemia which can result in severe consequences to the patient up to and including death. They include the following:                1) Not being able to immediately alert the patient and/or the physician of an ischemic event, particularly a life threatening event.        2) Not being ambulatory with the patient; and/or an inability to provide continuous monitoring to the patient and indication of the necessary diagnostic information to the physician.        3) Requiring input and interpretation of a physician or medical practitioner when one may not be present.        4) Requiring monitoring devices external to the body, such as an ECG monitor or external defibrillator, which are usually only available in medical centers and hospitals, and which further need special expertise and attention from medical personnel.        5) Reduced sensitivity or otherwise inability to detect ischemic events due to loss of sensitivity from use of external electrodes.        6) Loss of specificity as to the site of ischemia due to inadequate placement of electrodes in the vicinity of the ischemia or infarction.        7) Needing sophisticated expertise of a cardiologist to interpret the clinical condition or needing monitoring instruments with sophisticated computer-aided ECG signal analysis capabilities.        8) Over reliance on use of ECG signals for detection and inability to utilize and integrate other physiological data, (e.g. pressure, blood flow, and PO2).        9) Inability to immediately alert the patient or the physician of the impending or emerging ischemic condition.        10) Inability to provide immediate treatment, particularly for life-threatening events (e.g. myocardial infarction, cardiac arrest).        